1. Field of the Invention
The present invention relates to a color cathode-ray tube, and in particular to a color cathode-ray tube which is reduced in microphonic caused by vibration, deformation the at time of shadow mask assembling, and doming caused by thermal expansion of the shadow mask, and which displays images of favorable definition.
2. Discussion of the Background
In general, color cathode-ray tubes have an envelope including a substantially rectangular panel and a funnel. On the inner face of an effective portion formed by a curved face of the panel is formed a phosphor screen which is formed by three-color phosphor layers. On the inside of the phosphor screen, a substantially rectangular shadow mask is arranged to be opposed to the phosphor screen.
In the cathode-ray tube, three electron beams emitted from an electron gun disposed in a neck of the funnel are deflected by a deflection device mounted outside the funnel, and the phosphor screen is subject to horizontal and vertical scanning via the shadow mask. Thereby, a color image is displayed.
The shadow mask is provided to select the three electron beams incident on the three-color phosphor layers. In general, the shadow mask includes a nearly rectangular mask main body, and a nearly rectangular mask frame arranged along the periphery of the mask main body. The mask main body is formed by a curved face opposed to the phosphor screen. In addition, the mask main body includes a main surface portion having a large number of electron beam passage apertures, a nonporous portion located around the main surface portion, and a skirt portion located around the nonporous portion. The skirt portion is joined to side wall portions of the mask frame.
As for the combination of the mask main body and the mask frame, there are such a case that the side wall portions of the mask frame are joined inside the skirt portion of the mask main body, and such a case that the side wall portions are joined outside the skirt portion. Most large-sized tubes have such a structure that the side wall portions of the mask frame are attached outside the skirt portion.
In such a shadow mask, the distances between opposed open edges of the skirt portion in a shorter axis direction and a longer axis direction of the mask main body are set substantially equal to the distances between the side wall portions of the mask frame in the same directions.
In color cathode-ray tubes of recent years, it has been promoted to make an outer face of the effective portion of the panel a flat face or a curved face close to a flat face. In such color cathode-ray tubes, it is necessary to make the inner surface of the effective portion as well flat, as the outer surface of the effective portion is made flat. In the case where the inner surface of the effective portion of the panel is thus made flat, it is necessary to make the curvature of the main surface portion of the mask main body small and make the main surface portion flat or substantially flat, in order to make beam landing for the three-color phosphor layers favorable over the entire face of the screen.
If the curvature of the main surface portion of the mask main body becomes small, however, the tension strength of this main surface portion is lowered. If the color cathode-ray tube is incorporated into a television set in this case, then voice vibration fed from a speaker is transmitted to the mask main body. Because of resultant resonance of the mask main body, howling is apt to occur. The howling significantly degrades the image characteristics.
Furthermore, if the curvature of the main surface portion of the mask main body becomes small and the tension strength falls, then degradation of the color purity is apt to occur because of deformation of the mask main body caused in the manufacturing process of the color cathode-ray tube.
Furthermore, typically in color cathode-ray tubes, the quantity of the electron beams arriving at the phosphor screen via the electron beam passage apertures of the shadow mask is 1/3 or less of the electron beam quantity emitted from the electron gun, because of the operation principle. The rest of the electron beams mainly collide with the mask main body and heat it. Because of resultant thermal expansion of the shadow mask, such doming as to swell in the phosphor screen direction is caused in the mask main body. If the distance between the phosphor screen and the mask main body gets out of its tolerance due to the doming, the beam landing for the three-color phosphor layers deviates and color purity is degraded.
The shift amount of the beam landing caused by the doming largely differs depending upon the brightness of the image pattern, the duration of the pattern, and the like. Especially if a high brightness pattern is displayed locally, then local doming of the mask main body occurs, and local beam landing deviation occurs in a short time. And the local doming of the shadow mask appears especially largely in the case where the curvature of the main surface portion of the mask main body is small.